The present invention generally relates to measuring instruments for "standards room" or "measuring laboratory" accuracies, where measurements on the order of one hundred-thousandths (0.00001) of an inch are required. More specifically, the present invention relates to improvements for a cylindrical beam-type mechanical micrometer, such as the Supermicrometer Model A, manufactured by Pratt & Whitney.
Cylindrical beam-type mechanical micrometers have been used at least as early as 1924, when U.S. Pat. No. 1,490,413 was issued to B. H. Blood of the Pratt & Whitney Company. Basically, this type of micrometer is comprised of a cylindrical beam or bed of approximately four inches in diameter and two feet in length, having support legs at both ends so that it may rest upon a bench or table. At one end of the beam is a fixed-position head assembly, called a headstock, while moveably attached to the center portion of the beam is a tail assembly, called a tailstock. The headstock includes a measuring spindle with a calibrated dial, and the tailstock includes an anvil. The tailstock may be adjusted toward or away from the headstock to accommodate the length of the object to be measured, such that the length of the object falls within the one-inch measuring range of the spindle.
One particular brand of cylindrical beam-type micrometers have been widely used throughout the industry, namely, the Pratt & Whitney "Supermicrometer". Two basic models of the Supermicrometer were manufactured by Pratt & Whitney, i.e., the Mechanical Supermicrometer and the Electrolimit Supermicrometer. These two micrometers exhibit the same structural configuration except for the tailstock. The Mechanical Supermicrometer uses a mechanical pressure tailstock, while the Electrolimit Supermicrometer uses an electrical gauge in conjunction with an indicating meter to measure the tailstock pressure. Neither version of the Pratt & Whitney Supermicrometer has been available for some time. Nevertheless, the Pratt & Whitney Supermicrometer remains as one of the most widely used measuring instruments today.
The Mechanical Supermicrometer is a compact precision measuring instrument which is very similar to that shown in the aforementioned U.S. Pat. No. 1,490,413. It consists of a micrometer headstock with a precision measuring spindle that permits accurate measurements within its one-inch range of travel, and a mechanical pressure tailstock which provides a uniform measuring pressure of 1 pound or 21/2 pounds for all readings. Both the headstock and tailstock are mounted on a two-foot cylindrical beam, which provides a measuring range of 0-10 inches. The Mechanical Supermicrometer has an accuracy to 0.0001 inch (100 microinches).
The tailstock of the Electrolimit Supermicrometer is similar to that of the Mechanical Supermicrometer, in that it can be adjusted to allow measurements of objects of up to 10 inches in length. However, an electrical transducer is used in the tailstock anvil gauge head instead of a mechanical needle indicator. A portable displacement-measuring instrument cabinet is connected to the Electrolimit tailstock gauge head to indicate the correct measuring pressure when the instrument is used as a micrometer, and to give an indication of size when used as a comparator. When the gauge head armature is at the midpoint of its travel, a zero reading is indicated on the meter in the instrument cabinet. Any deviation of the tailstock anvil from this position will be indicated on the meter. Therefore, actual length measurements can be read directly from the headstock dial of the measuring spindle at the same accuracy as that of the Mechanical Supermicrometer, i.e., 0.0001 inches (100 microinches). When used as a comparator, the tailstock anvil is set at the desired measuring pressure, a zero size master is used to calibrate the meter at zero, and the deviation from the master size is read directly from the meter scale. When used in this manner, deviation in product size may be read directly from the meter to 0.00002 inches (20 microinches). Nonetheless, the Electrolimit Supermicrometer is still limited to measuring objects having a length of less than 10 inches.
In order to increase the measuring range capability of micrometers, various manufacturers, including Pratt & Whitney, generally abandoned the cylindrical beam-type Supermicrometer structure in favor of a long, flat, heavy, cast and seasoned iron bar-type measuring system. The flat bar-type structure rests on a precisely flat granite table, which provides the required stability to minimize influences on the integrity of the measuring system. Such bar-type systems have extended measuring ranges of up to 120 inches, but can obtain an accuracy of no better than 0.00005 inches (50 microinches). Due to the structural design changes in the flat bar-type measuring systems from the cylindrical beam-type micrometers, no parts are common between the two types of units. Hence, in order to measure objects of greater than 10 inches in length, a whole new measuring system must be purchased. Moreover, the cost of the extended-length flat bar-type systems is many times that of the Pratt & Whitney Supermicrometers.
Recently, at least one company has designed an upgrade package for the Pratt & Whitney Mechanical Supermicrometer to improve its accuracy. Optodyne, Inc., of Compton, Calif., offers a laser retrofit assembly for the headstock of the Supermicrometer. Using a modulated light beam from a helium-neon laser reflected off the measuring spindle of the Supermicrometer headstock, the laser retrofit package can improve the accuracy of the Mechanical Supermicrometer by at least a factor of ten, i.e., to an accuracy of within 0.00001 (10 microinches). Even with the laser retrofit package, however, the Supermicrometer is still limited to its original measuring range of 0-10 inches. Moreover, the laser requires at least a 60 minute warm-up time, and requires the use of an IBM-PC or compatible computer for certain types of calibration. Although the laser can be retrofit to the Supermicrometer, the laser retrofit package cannot be installed by the user, and is still too expensive for many small businesses.
A need, therefore, exists for a precision micrometer capable of measuring objects greater than ten inches in length to a high degree of accuracy at a relatively low cost.